The passage of Deuteronomy 21:15-17
is a specific instruction in the Law Itself to any man with
"two wives". If polygamy was a sin, then
it would not be possible for a "man to have two wives" in the Law.

"If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated:
Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn:
But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his."
Deuteronomy 21:15-17.